PROJECT SUMMARY/ABSTRACT Reasoning and envisioning personal future events (i.e., prospection) are associated with positive academic habits, healthy decision making, and psychological well-being. Despite the importance of these abilities, no study to date has examined the neural mechanisms underlying their development. The proposed project will directly address this gap in the literature. It builds on theory and empirical work with adults suggesting that memory integration supported by the anterior hippocampus (aHPC) and medial prefrontal cortex (mPFC) plays a critical role in both reasoning and prospection, as well as research showing that children have an immature aHPC-mPFC circuit and limitations in memory integration. We predict that reasoning and prospection in childhood may therefore rely on a more effortful strategy supported by the posterior HPC (pHPC) and lateral PFC (lPFC), wherein memories are kept distinct from one another and then recombined when necessary. Across two experiments, children (8-10 years) and adults (24-30 years) will complete tasks assessing memory integration, reasoning, and prospection. Memory integration and reasoning will be measured using a paired associative inference task (scanned), and prospection will be measured using a task in which participants provide narratives of envisioned personal future events (not scanned). Multimodal neuroimaging techniques (structural MRI, high-resolution and resting-state fMRI) and advanced analysis methods (multi-voxel pattern analysis) will be used to test experimental predictions. Results from this project would provide a substantial contribution to theory on the neurocognitive development of reasoning and prospection, and have the potential to provide critical insight into interventions designed to improve both of these abilities, especially within populations with deficits in memory, reasoning, or prospection (i.e., individuals with autism, depression, or schizophrenia).